Comment on ‘New Constraints on the P-T Evolution of the Alpe Arami Garnet Peridotite Body (Central Alps, Switzerland)' by Paquin & Altherr (2001)
7
0
0
Texte intégral
(2) JOURNAL OF PETROLOGY. VOLUME 42. Brey & Ko¨hler (1990), these temperatures yield a pressure of >6 GPa. Temperatures based on pyroxene ‘solvus’ relations (Brey & Ko¨hler, 1990) and on partitioning of other trace elements between Cpx Ib and Opx Ib yield much lower temperatures (>700–900°C). P&A conclude that whereas Grt achieved equilibrium with both Cpx and Opx for Fe, Mg, Ni, Co, Ca and Al, the two pyroxenes Ib, which are never found in contact with each other, attained equilibration only for relatively fastdiffusing elements, such as Co and Ni, but did not equilibrate in terms of Ca, V, Ti, Sc and Cr partitioning. Similar arguments are used to establish a rapid formation of pyroxene neoblasts (II) at high T, shortly after peak metamorphic conditions.. NUMBER 9. SEPTEMBER 2001. thermometers provide us with two independent evaluations of the closure temperature of the Ca–Mg exchange, which rules pyroxene solvus relations; the former essentially depends on the composition of the Cpx, the latter on that of the Opx. The temperatures given by the two thermometers diverge by <40°C at P = 3·2 GPa (Fig. 2). There is no indication of disequilibrium and therefore no reason to dismiss the low Cpx–Opx temperatures estimated by N&T and confirmed by a large number of samples, including those studied by P&A, Ernst (1978), Evans & Trommsdorff (1978) and Brenker & Brey (1997). The discrepancy between Tsolvus and TGrt–Opx,Cpx,Ol requires a different explanation.. Al in Opx and Cr in Cpx COMMENTS Ca partitioning between Opx and Cpx P&A highlight the existence of plateaux in the zoning profiles of Fe, Mg, Ni, Co and Al in the cores of pyroxene porphyroclasts and suggest that these reflect equilibration of Cpx and Opx with Ol and Grt at some P and T. They also compare the Ca contents of pyroxene Ib porphyroclasts and show that, despite their fairly flat profiles, they do not match the CaOpx/CaCpx systematics determined experimentally by Brey et al. (1990) on lherzolitic compositions at various P and T. They conclude that Cpx and Opx porphyroclast cores are not in equilibrium in terms of Ca partitioning, thus precluding the use of pyroxene solvus relations as a thermometer. However, their analysis is too simplified, as it does not take into account the role of other elements, which may have a strong influence on the partitioning of Ca. In particular, Na competes with Ca in Cpx M2 sites and thus has a major effect on the solubility of Ca. The ratio Ca/(1 − Na) is therefore a much more robust indicator of temperature than is Ca alone (Bertrand & Mercier, 1985; Brey & Ko¨hler, 1990). This issue is of crucial importance for Alpe Arami Cpx cores, which are characterized by high Na contents (0·134–0·151 a.p.f.u.) compared with Brey et al.’s experimental Cpx (<0·09 a.p.f.u.). Figure 1 shows that, when corrected for the effect of Na, the partitioning of Ca between Alpe Arami pyroxenes is consistent with the experiments and suggestive of T Ζ 900°C. It should be noted, however, that the Ca/(1 − Na) ratio is also slightly composition dependent and its use as a thermobarometer is not straightforward. A more robust approach, already tackled by N&T, would have been to compare the temperatures yielded by the pyroxene solvus thermometer of Brey & Ko¨hler (1990) or Taylor (1998), with those yielded by the Cain-Opx thermometer of Brey & Ko¨hler (1990). The two. P&A rightly note that ‘Al contents in the cores of pyroxene Ib porphyroclasts are nearly constant and similar to those of pyroxene Ia inclusions in garnet’, suggesting ‘equilibration with garnet at some P and T ’. Therefore they deem the Opx–Grt Al-exchange barometer suitable for an evaluation of P. However, P&A do not take into due consideration the fact that also Cr exhibits flat profiles in both Cpx Ib porphyroclasts and Cpx Ia inclusions in garnet and that the contents of this element are virtually identical in the two textural types (see their fig. 5). Given the very low diffusivity of Cr, the flat profiles also suggest complete equilibration of Cpx with Grt, the other main Cr reservoir, at some P and T. The Cr-in-Cpx barometer of Nimis & Taylor (2000), which is based on Cr partitioning between Cpx and Grt, is therefore also suitable for an assessment of P. Both the Opx–Grt and the Cr-in-Cpx barometers are virtually insensitive to the composition of the garnet. In particular, the Cr-in-Cpx barometer has been widely tested on mantle xenoliths and diamond inclusions against the graphite–diamond curve (Nimis & Taylor, 2000). The Taylor (1998) version of the Opx–Grt barometer is preferred here to the more popular Brey & Ko¨hler (1990) one because it yields estimates almost identical to those yielded by the Cr-inCpx barometer when applied to well-equilibrated peridotites (Nimis & Taylor, 2000). The Opx–Grt and Crin-Cpx barometers show a very good agreement when applied to P&A pyroxenes assuming a nominal temperature equal to Tsolvus, but they diverge substantially at higher temperatures (Fig. 2). The use of the Brey & Ko¨hler (1990) version of the Opx–Grt barometer would even enhance this divergence.. Al zoning in pyroxenes P&A interpret the approximately M-shaped Al zoning profile in Opx Ib porphyroclasts as a result of decompression followed by cooling. They also assert that. 1774.
(3) NIMIS AND TROMMSDORFF. COMMENT. Fig. 1. Ca partitioning corrected for Na between clinopyroxenes and orthopyroxenes in garnet peridotites from Alpe Arami (Ia, Ib and II; Paquin & Altherr, 2001) and other Central Alps occurrences (Nimis & Trommsdorff, 2001), and in experiments on lherzolite compositions (Brey et al., 1990). Core–rim trends are shown where significant as arrows. Thick and thin lines are respectively approximate isobars (GPa) and isotherms based on the experiments. It should be noted that, because of compositional factors, the use of Ca/(1 − Na) as a thermobarometer is not straightforward.. Al zonation in Cpx Ib porphyroclasts is characterized by ‘continuously decreasing Al concentrations from core to rim, suggesting somewhat different P–T evolution’ and therefore supporting their ‘interpretation that both pyroxenes Ib were not in chemical equilibrium with respect to Ca and Al’. However, Na in Cpx Ib similarly shows a rimward decrease (see P&A, fig. 5), suggesting that Al content in Cpx has at least in part been controlled by jadeite-consuming reactions, rather than exclusively by Cpx–Grt equilibria. Similar trends are common in partially retrogressed garnet peridotites (e.g. Evans & Trommsdorff, 1978; Medaris, 1984; Nimis & Morten, 2000) and can be explained in terms of net transfer reactions of the type jadeite + 2diopside + (3/2)enstatite + H2O → edenite, related to kelyphite and amphibole neoblast formation. Therefore, differences in the Al profiles in Cpx and Opx may simply reflect the higher sensitivity of AlOpx to P–T variations during the garnetstage and the higher sensitivity of (Na,Al)Cpx to retrogressive, post-garnet reactions. Relatively flat Na plateaux are anyway still preserved in the cores of the Cpx Ia and Ib, suggesting maintenance of the original equilibrium compositions. It is worth noting that for Cpx Ia inclusions in garnets, which were better preserved from hydration reactions, no kelyphite is typically observed at the Grt– Cpx Ia interface and they show an M-shaped profile that. mimics that of Opx. The close similarity between the two profiles becomes apparent when figs 3 and 5 of P&A are reset to the same vertical scale. The apparently more complex zoning profiles in Cpx II neoblasts simply reflect the greater sensitivity of smaller Cpx grains to amphiboleproducing reactions. [It is worth noting that jadeite loss has little effect on P calculations using the Cr-in-Cpx barometer of Nimis & Taylor (2000) and on T calculations using pyroxene solvus thermometers. Therefore their application to Cpx II neoblasts remains warranted.] There is therefore little reason to assign Cpx and Opx different P–T histories based on Al relations. It is also worth noting here that the initial rimward increase of Al in Opx (and in Cpx Ia) can also be interpreted as a result of isobaric heating, rather than decompression. The relative implications will be discussed below.. The high-T stage The strongest argument put forward by P&A to support their high-T scenario is the finding of Mg- and Ni-rich cores in the garnets (see also Brenker & Brey, 1997). Assuming these cores are in equilibrium with olivine and pyroxenes, their composition implies high temperatures (>1100–1200°C; Fig. 2) and the observed rimward decrease of Mg and Ni indicates rapid cooling (see Paquin. 1775.
(4) JOURNAL OF PETROLOGY. VOLUME 42. NUMBER 9. SEPTEMBER 2001. Fig. 2. Thermobarometry of P&A samples. P and T are calculated using mineral core compositions extracted from P&A’s zoning profiles. Continuous lines refer to slow-diffusing element equilibria (Ca–Mg, Cr, Al); dashed lines refer to fast-diffusing element equilibria (Fe–Mg, Ni, Co). The low-T stage is ascribed to high-P metamorphism during Alpine subduction at 43 Ma. The high-T stage is ascribed to a short-lived event related to magmatic intrusion at 35 Ma. Accurate barometry of the high-T stage is not practicable because the thermometers intersect at low angle and because of probable partial re-equilibration under cooling, but the most consistent temperatures are obtained for pressures not very dissimilar from those attained during the low-T stage. Uncertainties for thermometers and barometers are probably within 50–70°C and 0·25 GPa, respectively. NT00, Nimis & Taylor (2000; Cr-in-Cpx); TA98, Taylor (1998; Opx–Grt; Al); BK90 Cpx, Brey & Ko¨hler (1990; Cpx–Opx; Ca–Mg); BK90 Opx, Brey & Ko¨hler (1990; Ca-in-Opx); HA84, Harley (1984; Opx–Grt; Fe–Mg); KR00, Krogh Ravna (2000; Cpx–Grt; Fe–Mg); SAL99, Seitz et al. (1999; Cpx–Opx; Co); Ni-in-Grt, Canil (1999; Ol–Grt; Ni); OW79, O’Neill & Wood (1979; Ol–Grt; Fe–Mg). The original HA84 thermometer has been empirically corrected to make up for its inconsistency at very low and very high T (see Brey & Ko¨hler, 1990; Nimis & Trommsdorff, 2001).. & Altherr, 2000). Using the expression h = 8(D.t), based on Fick’s second law, and assuming a diffusion coefficient (D) of >10−14 cm2/s for the Fe–Mg couple in Grt at 1150°C and 3 GPa (Chakraborty & Ganguly, 1991), the estimated time required to completely equilibrate a 3·5 mm garnet porphyroblast would be <0·1 m.y. The diffusivity of Ni in Grt is comparable with that of the Fe–Mg couple (Griffin et al., 1996). The diffusivities of the Ca–Mg couple in Cpx and of Al in both pyroxenes are two to three orders of magnitude lower and those of Cr can be even less (Sautter et al., 1988; Smith & Barron, 1991; Dimanov & Sautter, 2000). A relatively short-lived thermal pulse could thus cause complete re-equilibration. of the garnets in terms of Fe–Mg and Ni exchanges, although leaving the composition of the pyroxene cores virtually unaffected. Therefore, the high TGrt–Opx,Cpx,Ol calculated by P&A do not necessarily contradict the low-T estimates indicated by pyroxene solvus relations. Rather, the two temperatures may reflect distinct stages in the metamorphic history of the Alpe Arami peridotite. Opx–Grt TFe–Mg for Opx Ia inclusions in garnets are slightly lower than those calculated for the Opx Ib porphyroclasts (Fig. 2). This difference may reflect the greater susceptibility of small Opx inclusions to partial re-equilibration under cooling after the above thermal pulse. The apparent lack of Fe–Mg re-equilibration in. 1776.
(5) NIMIS AND TROMMSDORFF. Cpx Ia inclusions after the thermal pulse is consistent with the much lower Fe–Mg diffusivity in Cpx relative to that in Opx and Grt (Dimanov & Sautter, 2000). The relatively low Cpx–Grt TFe–Mg obtained for Cpx II neoblasts probably reflects incomplete Grt–Cpx II equilibration during the high-T stage. Because the solubilities of Al in Opx and Cpx coexisting with Grt, and of Ca in Opx coexisting with Cpx, increase significantly with rise in temperature, the short-lived thermal event is also the most probable source of the approximately M-shaped zoning profiles of Al in both Opx and Cpx Ia and of Ca in Opx (see P&A, figs. 3 and 5). Although less distinct, Cpx consistently exhibits complementary W-shaped Ca profiles. Furthermore, P&A admit they have no convincing explanation for the peculiar W-shaped Cr profiles in Grt (see P&A, fig. 2). These can be related to reactions of the type (2/3)uvarovite + (1/3)knorringite + (2/3)grossular + (1/3)pyrope → 2 CaCrAlSiO6 + 2 diopside (see Nimis & Taylor, 2000), which reflect either decompression or a rise in T, and are therefore also consistent with a thermal pulse followed by cooling. Significantly, a complementary profile is exhibited by Cpx Ia inclusions, which show a tenuous, yet appreciable, Cr maximum >100 mm from the grain edges. Incidentally, we recall that P&A reiterate the use of Ca/Cr relations in garnet made by Brenker & Brey (1997) to constrain P–T estimates. This issue had already been discussed extensively by N&T, who demonstrated the inconsistency of the thermobarometric formulations of Brenker & Brey on the basis of experimental data on lherzolitic compositions. This point needs no further discussion.. Opx–Cpx trace element equilibria The Opx–Cpx thermometer of Seitz et al. (1999) based on Co partitioning between Opx and Cpx yields high temperatures (1000–1100°C) when applied to P&A’s pyroxenes. These temperatures are similar to those yielded by Fe–Mg and Ni exchange thermometry and are in line with the relatively high expected diffusivity of Co (see P&A). As for other trace elements, the Seitz et al. Sc, V and Cr thermometers yield very low temperatures (<750°C at 3·2 GPa). We agree with P&A that some degree of disequilibrium may have occurred between the pyroxenes at peak metamorphic conditions for these slow-diffusing elements. We just add that uncertainties in the determinations of trace element abundances, especially in Opx, may also have constituted a significant source of error. Finally, the finding of Li enrichments in Alpe Arami Cpx (Paquin et al., 2000) cannot be invoked as a proof for general disequilibrium between the pyroxenes, as Li-enriched Cpx in cryptically metasomatized. COMMENT. peridotites may well maintain equilibrium major-element compositions (Seitz & Woodland, 2000).. CONCLUSIONS Compositional data and zoning patterns in pyroxenes offer no valid reason to consider early-generation Cpx and Opx cores out of equilibrium, at least in terms of Ca–Mg and Al partitioning. Then the low temperatures estimated on the basis of pyroxene solvus relations pose a challenging problem to the high temperatures calculated from Fe–Mg and Ni thermometry of Grt–Ol and Grt– pyroxene pairs. We are thus left with three possibilities: (1) all Grt-based temperatures are wrong; (2) pyroxene solvus-based temperatures are wrong; (3) Grt-based and pyroxene solvus-based temperatures reflect compositional relations that were produced at different stages and under different P–T conditions. The first hypothesis was strongly favoured by us in N&T. In that paper we suggested that garnet core composition could in part have been controlled by local equilibria with pre-existing spinel grains, hence precluding the use of garnet cores to calculate P–T conditions. The high sensitivity of some thermometers to errors in the determination of equilibrium compositions, including those derived from analytical uncertainties, and propagation of errors on P estimates, would have produced the anomalously high T estimates. None the less, the remarkable agreement between the Ol–Grt, Opx–Grt and Cpx–Grt TNi and TFe–Mg estimated by P&A and, particularly, the shapes of Mg, Fe and Ca zoning profiles in garnets, which do not support the hypothesis of growth zoning on spinel sites, weaken the arguments originally put forward by N&T. The second hypothesis was contended by P&A, but their arguments have been shown here to be inconclusive and, in part, incorrect. In any case, even rejecting solvusbased temperatures, the poor accord between the two independent Opx–Grt and Cpx–Grt barometers at high T renders P&A’s barometric data for peak metamorphic conditions unreliable and suggests that they can have been significantly overestimated (Fig. 2). Accurate barometry of the high-T stage is not anyway practicable, both because the thermometers involved intersect at a low angle and because of probable partial garnet and pyroxene Fe–Mg, Ni and Co re-equilibration upon cooling. If P&A’s pressure estimates were too high, their Grt–(Ol, Cpx, Opx) TMg–Fe calculated at P = 6 GPa may also be excessive. Using the nominally P-independent Ni-in-Grt thermometer, minimum temperatures of >1100°C can be estimated for the high-T stage. We believe the third hypothesis to be the most rational, as it reconciles both the high- and the low-temperature. 1777.
(6) JOURNAL OF PETROLOGY. VOLUME 42. estimates obtained for different chemical equilibria. Following the same line of reasoning as used by P&A, the origin of the discrepancy in thermometric data must lie in the different diffusivities of the various cations exchanged. As Ca–Mg, Cr and Al in pyroxenes diffuse much more slowly than Fe–Mg and Ni in garnets, the low-T conditions recorded by pyroxene Ca–Mg equilibria must pre-date the thermal event responsible for the high Fe–Mgand Ni-exchange temperatures. For the same reason, the barometric data extracted from pyroxene–garnet Cr and Al partitioning data should not be attributed to the highT stage, but to the earlier low-T stage. This is corroborated by the good agreement between pressure estimates obtained using the Cpx–Grt and Opx–Grt barometers at T given by pyroxene solvus thermometry for all pyroxene generations (Ia, Ib, II). The corresponding P–T conditions of >800°C and >3 GPa (Fig. 2) fit the Alpine, prograde metamorphic sequence of the Adula–Cima Lunga unit and can therefore be assumed as representative of the peak metamorphic conditions reached by the Alpe Arami peridotite during Alpine subduction dated at 43 Ma by Gebauer (1996) on metamorphic zircons in Grtpyroxenites, eclogites and Grt-peridotites. These moderate P–T conditions are also in line with the fact that unique early features are shared by the Alpe Arami and Cima di Gagnone garnet peridotites, the latter pointing to an upper pressure limit around 3 GPa and temperatures around 750°C (Nimis & Trommsdorff, 2001). These features are ilmenite rods and palisades (Risold et al., 2001) and identical, metamorphic crystal preferred orientations with [100] perpendicular to foliation in first-generation olivine at both localities (Frese et al., 2001). The short-lived thermal event at T [ 1100°C can be related to magmatic intrusions, which have been dated at 35 Ma by Gebauer (1996) on magmatic zircons in Grt-pyroxenites, eclogites and Grtperidotites. [Gebauer (1996) proposed an origin of the magmatic zircons from melts produced by decompressiondriven partial melting of the Alpe Arami peridotite. An exotic origin of the melts seems, however, more consistent with the thermobarometric data.] Accurate barometry of this high-T phase is not anyway practicable on the basis of available data, because of lack of high-T pyroxene–garnet equilibrium pairs. Most likely, pressure conditions were not very dissimilar from those recorded at the peak of the subduction metamorphism (Fig. 2). As a working hypothesis, we suggest a causal correlation between the heating event at 35 Ma and the magma-driven Li metasomatism described by Paquin et al. (2000).. ACKNOWLEDGEMENTS We are grateful to Jens Paquin for continuing and stimulating discussion and for supplying a preprint version of the P&A paper. Paolo Nimis gratefully acknowledges. NUMBER 9. SEPTEMBER 2001. financial support of MURST ex 60%, ‘Progetti di Ricerca Giovani Ricercatori’ (Padova) and C.N.R. ‘Centro di Studio per la Geodinamica Alpina’ (Padova).. REFERENCES Becker, H. (1993). Garnet peridotite and eclogite Sm–Nd mineral ages from the Lepontine dome (Swiss Alps): new evidence for Eocene high-pressure metamorphism in the central Alps. Geology 21, 599–602. Bertrand, P. & Mercier, J.-C. C. (1985). The mutual solubility of coexisting ortho- and clinopyroxene: toward an absolute geothermometer for the natural system? Earth and Planetary Science Letters 76, 109–122. Brenker, F. E. & Brey, G. P. (1997). Reconstruction of the exhumation path of the Alpe Arami garnet-peridotite body from depths exceeding 160 km. Journal of Metamorphic Geology 15, 581–592. Brey, G. P. & Ko¨hler, T. (1990). Geothermobarometry in four-phase lherzolites II. New thermobarometers, and practical assessment of existing thermobarometers. Journal of Petrology 31, 1353–1378. Brey, G. P., Ko¨hler, T. & Nickel, K. G. (1990). Geothermobarometry in four-phase lherzolites I. Experimental results from 10 to 60 kb. Journal of Petrology 31, 1313–1352. Canil, D. (1999). The Ni-in-garnet geothermometer: calibration at natural abundances. Contributions to Mineralogy and Petrology 136, 240– 246. Chakraborty, S. & Ganguly, J. (1991). Compositional zoning and cation diffusion in garnets. In: Ganguly, J. (ed.) Diffusion, Atomic Ordering, and Mass Transport. Advances in Physical Geochemistry, 8. Berlin: Springer, pp. 120–175. Dimanov, A. & Sautter, V. (2000). ‘Average’ self-diffusion of (Fe, Mn)–Mg in natural diopside. European Journal of Mineralogy 12, 749– 760. Dobrzhinetskaya, L., Green, H. W. & Wang, S. (1996). Alpe Arami: a peridotite massif from depths of more than 300 kilometers. Science 271, 1841–1845. Ernst, W. G. (1978). Petrochemical study of the lherzolitic rocks from the Western Alps. Journal of Petrology 19, 341–392. Evans, B. W. & Trommsdorff, V. (1978). Petrogenesis of garnet lherzolite, Cima di Gagnone, Lepontine Alps. Earth and Planetary Science Letters 40, 333–348. Frese, K., Trommsdorff, V., Wenk, H. R. & Kunzer, K. (2001). Metamorphic origin of olivine [100] CPO’s. Abstracts of the International Conference on Deformation Mechanisms, Rheology and Tectonics. Utrecht: University of Utrecht, p. 53. Gebauer, D. (1996). A P–T–t path for an (ultra?-) high-pressure ultramafic/mafic rock-association and its felsic country-rocks based on SHRIMP-dating of magmatic and metamorphic zircon domains. Example: Alpe Arami (Central Swiss Alps). In: Basu, A. & Hart, S. (eds) Earth Processes: Reading the Isotopic Code. Geophysical Monograph, American Geophysical Union 95, 307–329. Gebauer, D., Gru¨nenfelder, M., Tilton, G., Trommsdorff, V. & Schmid, S. (1992). The geodynamic evolution of garnet-peridotites, garnetpyroxenites and eclogites of Alpe Arami and Cima di Gagnone (Central Alps) from Early Proterozoic to Oligocene. Schweizerische Mineralogische und Petrographische Mitteilungen 72, 107–111. Green, H. W., Dobrzhinetskaya, L., Riggs, E. M. & Jin, Z. M. (1997). Alpe Arami: a peridotite massif from the mantle transition zone. Tectonophysics 279, 1–21. Griffin, W. L., Smith, D., Ryan, C. G., O’Reilly, S. Y. & Win, T. T. (1996). Trace-element zoning in mantle minerals: metasomatism and thermal events in the upper mantle. Canadian Mineralogist 34, 1179–1193.. 1778.
(7) NIMIS AND TROMMSDORFF. Harley, S. L. (1984). An experimental study of the partitioning of Fe and Mg between garnet and orthopyroxene. Contributions to Mineralogy and Petrology 86, 359–373. Krogh Ravna, E. (2000). The garnet–clinopyroxene Fe2+–Mg geothermometer: an updated calibration. Journal of Metamorphic Geology 18, 211–219. Medaris, L. G. (1984). A geothermobarometric investigation of garnet peridotites in the Western Gneiss Region of Norway. Contributions to Mineralogy and Petrology 87, 72–86. Nimis, P. & Morten, L. (2000). P–T evolution of ‘crustal’ garnet peridotites and included pyroxenites from Nonsberg area (Upper Austroalpine), NE Italy: from the wedge to the slab. Journal of Geodynamics 30, 93–115. Nimis, P. & Taylor, W. R. (2000). Single clinopyroxene thermobarometry for garnet peridotites. Part I. Calibration and testing of a Cr-in-Cpx barometer and an enstatite-in-Cpx thermometer. Contributions to Mineralogy and Petrology 139, 541–554. Nimis, P. & Trommsdorff, V. (2001). Revised thermobarometry of Alpe Arami and other garnet peridotites from the Central Alps. Journal of Petrology 42, 103–115. O’Neill, H. St. C. & Wood, B. J. (1979). An experimental study of Fe–Mg-partitioning between garnet and olivine and its calibration as a geothermometer. Contributions to Mineralogy and Petrology 70, 59–70. With corrections by O’Neill, H. St. C. (1980). Contributions to Mineralogy and Petrology 72, 337. Paquin, J. & Altherr, R. (2000). Modelling the exhumation path of the UHP-garnet peridotite from Alpe Arami. EOS Transactions, American Geophysical Union, Supplement 81, F1104.. COMMENT. Paquin, J. & Altherr, R. (2001). New constraints on the P–T evolution of the Alpe Arami garnet peridotite body (Central Alps, Switzerland). Journal of Petrology 42, 1119–1140. Paquin, J., Altherr, R. & Vennemann, T. W. (2000). Li metasomatism during exhumation of the ultra high-pressure garnet peridotite of the Alpe Arami, Central Alps (Switzerland). Journal of Conference Abstracts 5, 771. Risold, A. C., Trommsdorff, V. & Grobe´ty, B. (2001). Genesis of ilmenite rods and palisades along humite-type defects in olivine from Alpe Arami. Contributions to Mineralogy and Petrology 140, 608–618. Sautter, V., Jaoul, O. & Abel, F. (1988). Aluminum diffusion in diopside using the 27Al, 28Si nuclear reaction: preliminary results. Earth and Planetary Science Letters 89, 109–114. Seitz, H. M. & Woodland, A. B. (2000). The distribution of lithium in peridotitic and pyroxenitic mantle lithologies—an indicator of magmatic and metasomatic processes. Chemical Geology 166, 47–64. Seitz, H. M., Altherr, R. & Ludwig, T. (1999). Partitioning of transition elements between orthopyroxene and clinopyroxene in peridotitic and websteritic xenoliths: new empirical geothermometers. Geochimica et Cosmochimica Acta 63, 3967–3982. Smith, D. & Barron, B. R. (1991). Pyroxene–garnet equilibration during cooling in the mantle. American Mineralogist 76, 1950–1963. Taylor, W. R. (1998). An experimental test of some geothermometer and geobarometer formulations for upper mantle peridotites with application to the thermobarometry of fertile lherzolite and garnet websterite. Neues Jahrbuch fu¨r Mineralogie, Abhandlungen 172, 381–408.. 1779.
(8)
Figure
Documents relatifs
Coastal neotectonics in Southern Central Andes: uplift and deformation of marine terraces in Northern Chile (27 degrees S).. Quaternary Sea Level Events on Northern San
Magnesite is widespread in New Caledonia and occurs as veins within the serpentine sole 67.. of the Peridotite Nappe, and as nodular heaps in recent alluvial deposits and
We showed here that CV chondrites (CV Red and CV Ox ) are hydrated to a variable extent that is controlled by the peak metamorphic temperature. Based on the total
C’est pour cette raison que malgré tout ce que j’ai lu et malgré l’énorme envie de lui et les sentiments envers cet homme, il resta et restera un ami spécial oui mais seulement
Sa pâleur désunit nos deux ombres mêlées Et les rires joyeux de nos douces soirées, Il s’en va réchauffer un autre amour naissant.. Sa pâleur désunit nos deux ombres mêlées,
[r]
Toutefois, l’œuvre de l’écrivain, comme celle de nombre de ses contemporains français, ne fait pas simplement table rase des expérimentations du nouveau roman et du
Based on detailed documentation in the Ronda peridotite, and considering isotherms geometry and foliation tracks in the mantle wedges of the Ryukyu and Honshu subduction systems,
